NAME

       radioclkd - decode time from radio clock(s) attached to serial port

SYNOPSIS

       radioclkd [ -tphv ] device

DESCRIPTION

       radioclkd  is  a simple daemon that decodes the time from a radio clock device attached to
       the DCD and/or CTS and/or DSR status lines of serial port of a computer.  It  is  able  to
       decode  the DCF77, MSF and WWVB time signals. The received time is then sent to ntpd using
       the shared memory reference clock driver. The  type  of  time  signal  being  received  is
       automatically  determined.  If  you  have  problems  getting  the  program  to  work using
       interrupts, the following command is known to help in many instances. If  this  fails  you
       can always fall back to the polling method.

              stty crtscts < /dev/ttyS0

       Details  on  a cheap and easy to make device for receiving these time signals can be found
       at

              http://www.buzzard.org.uk/jonathan/radioclock.html

OPTIONS

       -p, --poll
              Poll the serial port for changes of status in the DCD, CTS  and  DSR  lines  rather
              than use interrupts

       -t, --test
              Enter  test  mode printing the length of each pulse and the decoded time at the end
              of each minute on stdout. The time is not sent to  ntpd  using  the  shared  memory
              reference clock driver in this mode.

       -h, --help
              Print a short synopsis of the command line arguments.

       -v, --version
              Print the version number and then exit.

CONFIGURATION

       Configuration  is  very  simple. Use server 127.127.28.0 in your ntp.conf file for a clock
       attached to the DCD line, server 127.127.28.1 for a clock attached to the  CTS  line,  and
       server  127.127.28.2  for  a  clock  attached to the DSR line. You will also want to use a
       fudge line on the server to change the displayed refid.

CALIBRATION

       Due to delays in the propogation of the radio signal,  it's  processing  by  the  receiver
       board  and  the  latency  of the operating system the time decoded by the receiver will be
       slightly offset from actual UTC. Typically this delay will be less than  20ms,  so  unless
       you  are  very fussy about the time, or are using more than one time source, such as a GPS
       unit, other radio clock or NTP server on the internet you can ignore this section.

       The basics of the calibration procedure is to determine the average offset  of  the  radio
       receiver,  and  use  the  time1 fudge factor in ntp.conf to bring the receiver as close as
       possible to the real time. The  easiest  way  of  determining  the  offset  of  the  radio
       receivers  time  is  to  run  it against a reference clock that does not suffer from these
       problems. The best reference clock would be a GPS unit. This might be a GPS unit that  you
       don't  wish  to  dedicate to time keeping, or a borrowed unit. If this is not possible you
       could use a stratum 1 server on the internet.

       The method of calibration is quite simple. We attach the calibration  reference  clock  to
       the  computer and fudge the stratum of our radio receiver up to say 5.  This way we can be
       sure that ntpd will lock onto the calibration reference clock. We need to make  sure  that
       ntpd  is  configured to collect peer statistics so make sure we have some lines similar to
       these in ntp.conf

           statsdir /var/log/ntpstats/
           statistics loopstats peerstats clockstats
           filegen peerstats file peerstats type day enable

       After that we restart ntpd and leave it running for several hours. We can then make a copy
       the peerstats file. The trick is to remove all the entries before ntpd has come into close
       aggrement with the calibration reference clock and then run the  peer.awk  script  in  the
       scripts/stats  directory  of  the ntp distribution. This will give us a mean offset of our
       radio receivers in milliseconds. This can them be converted into seconds and added to  the
       fudge line in ntp.conf for our receiver.

       The  final  step  is  to  remove  the  change in stratum level for our reference clock and
       restart ntpd. If you move the receiver any significant distance  then  you  will  need  to
       repeat this calibration step. Across the room or around the current building will be fine,
       but if you move it to the next town/city then you will need to recalibrate.

IN USE

       The version of ntpd that comes with most Linux distributions  does  not  have  the  shared
       memory  reference  clock driver compiled in by default. This can be identified by checking
       the logs after ntpd is started. If  the  shared  memory  reference  clock  driver  is  not
       compiled in then the logs will contain warnings about the reference clock driver not being
       recognized. To compile ntpd with the shared memory reference clock driver you must specify
       the --enable-SHM option when running configure.

       Neither  radioclkd  or  ntpd ever mark the shared memory segment for deletion. If you stop
       using the shared memory reference clock driver therefore any shared memory  segments  will
       persist  until you reboot or manually delete the segment using ipcrm.  The segments can be
       identified as the one with  key  0x4e545030,  0x4e545031  or  0x4e545032  using  the  ipcs
       command.

BUGS

       If you are running a kernel with the PPS kit and have a clock attached to the DCD line you
       may experience lockups. If you encounter this problem the currently recomended solution is
       to move the clock to either the CTS or DSR lines.

AUTHOR

       This  program  was written by Jonathan Buzzard <jonathan@buzzard.org.uk> and may be freely
       distributed under the terms of the GNU General Public  License.  There  is  ABSOLUTELY  NO
       WARRANTY for this program.